I am trying to clear arp-cache in IOS 12.1 and get unexpected results. Below
you see debug arp turned on, the current arp cache, then the clear command.
Immediately, the router sends out arp requests for the entries that were in
the cache.
Why does the router immediately send arp requests when I attempt to clear
the cache? I would think it would wait till a need arose to send the
appropriate arp request, afterall, I do want to clear the cache? Is there
some way to prevent this?
Further, looking at the debug #1 and 2, why is the arp request being sent
with target MAC address? This is more confusing. Why bother arping if it
knows the target MAC address? I can only think Cisco is saving some
bandwidth here.
And a bigger question, why is the router sending itself an ARP...packet
number 3 below. Is it some sort of security measure? Does it do this at
startup too?
Thanks.
Router#debug arp
ARP packet debugging is on
Router#show arp
Protocol Address Age (min) Hardware Addr Type Interface
Internet 192.168.0.101 - 00d0.0b83.da30 ARPA Ethernet0
Internet 192.168.0.4 3 00d1.ab8a.3e8f ARPA Ethernet0
Internet 192.168.0.5 3 00e1.28a1.080d ARPA Ethernet0
Router#clear arp-cache
Router#
02:32:33: IP ARP: sent req src 192.168.0.101 00d0.0b83.da30,
dst 192.168.0.4 00d1.ab8a.3e8f Ethernet0
02:32:33: IP ARP: sent req src 192.168.0.101 00d0.0b83.da30,
dst 192.168.0.5 00e1.28a1.080d Ethernet0
02:32:33: IP ARP: sent rep src 192.168.0.101 00d0.0b83.da30,
dst 192.168.0.101 ffff.ffff.ffff Ethernet0
02:32:33: IP ARP: rcvd rep src 192.168.0.4 00d1.ab8a.3e8f, dst 192.168.0.101
Ethernet0
02:32:33: IP ARP: creating entry for IP address: 192.168.0.4, hw:
00d1.ab8a.3e8f
02:32:33: IP ARP: rcvd rep src 192.168.0.5 00e1.28a1.080d, dst 192.168.0.101
Ethernet0
Router#
02:32:33: IP ARP: creating entry for IP address: 192.168.0.5, hw:
00e1.28a1.080d
Router#
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